Friday, August 30, 2013
Cold storage: A freeze wall created for a construction project by the company SoilFreeze.
Japanese officials desperate to contain an ever-growing crisis at the Fukushima nuclear power station are looking to use artificial permafrost to stop radioactive water from leaking. The idea is to build a mile-long wall of frozen earth around Fukushima's toxic reactor buildings to stem the groundwater contamination; the most experienced specialists in the field say the plan should work.
The Japanese firms involved appear to be taking a go-it-alone approach. Two weeks ago, a top official at Tokyo Electric Power (Tepco) signaled that the utility behind the Fukushima disaster would seek international assistance with the Fukushima water contamination crisis. But experts at U.S.-based firms and national labs behind the world's largest freeze-wall systems—and the only one proven in containing nuclear contamination—have not been contacted by either Tepco or its contractor, Japanese engineering and construction firm Kajima Corp.
One of these experts is Elizabeth Phillips, who managed the installation of a 300-foot-long, 30-foot-deep freeze wall to isolate radioactive waste at the U.S. Department of Energy's Oak Ridge National Laboratory in Tennessee in 1996 and 1997. While freeze walls are commonly used to hold back groundwater to facilitate excavations at construction sites and mines, this case calls for specialized expertise, she says. "You need to make sure that whoever is doing it is analyzing everything that can go wrong," says Phillips. "You should go with someone who has done it before."
Every day roughly 400 tons of groundwater flowing down from the nearby mountains enters cracks in the reactor buildings damaged by the meltdowns and explosions at Fukushima in 2011, according to an April 2013 Tepco briefing document. Water that escapes from the buildings pollutes the groundwater downstream and ultimately spills into the sea. The contaminant levels are dangerously high. Last month Tepco pulled water from a sampling well downstream of the buildings containing radiation levels that were orders of magnitude higher than the levels deemed safe by the Japanese Nuclear Regulation Authority.
Tepco's efforts to prevent that spread so far have been ineffective, risky, and ultimately unsustainable. Its primary response has been to pump contaminated groundwater into holding tanks, adding to the more than 300,000 tons of radioactive water already stored at Fukushima in hastily assembled tanks that are vulnerable to future earthquakes. Some have already leaked. Last week Japan's Nuclear Regulation Authority recorded one recent 300-ton leak as a level-3 incident—the first incident at Fukushima that it has rated on the international nuclear event scale since 2011.
The freeze wall would be a more definitive approach to managing groundwater. As proposed by Kajima in April and endorsed in May by a Nuclear Regulation Authority expert panel, it would run 1.4 kilometers and encircle the site's four destroyed reactors. Vertical pipes are to be drilled or driven into the ground at one-meter intervals, creating what looks like an array of sub-soil fence posts. Fourteen 400-kilowatt refrigeration plants would pump -20 °C to -40 °C coolant down each pipe to absorb heat from the ground, producing an expanding cylinder of frozen earth.
In roughly six weeks, those cylinders would fuse together to form a continuous barrier that keeps groundwater out and contaminants in. The result would be a solid barrier from the surface extending approximately 95 feet down to meet a low-permeability layer of clay and rock. And while it would require long-term chilling to endure, the wall is immune to power outages lasting days or weeks. "It would take months or years to thaw the wall out," says Daniel Mageau, vice president and design engineer for Seattle-based contractor SoilFreeze.
Several features make freeze walls better barriers than those fashioned from steel, concrete, or clay—alternatives that the Nuclear Regulation Authority's panel considered and rejected. A key advantage cited by Phillips is the freeze wall's self-healing capacity. For example, water flowing into cracks caused by an earthquake—an ever-present threat at Fukushima—would freeze to reëstablish the barrier. "That's a really great asset," says Phillips.
The Oak Ridge experience suggests it will work at Fukushima, according to Phillips and experts at the contractors that built the lab's wall: Rockaway, New Jersey-based geotechnical contractor Moretrench and Anchorage-based Arctic Foundations. It remains the only nuclear containment freeze wall project to date, and one that has been mischaracterized in press reports as an experiment. "It wasn't a model. It was drilling into contaminated soils and stopping real radiologically contaminated materials from escaping and traversing down into a creek," says Arctic Foundations chief engineer Edward Yarmak.
The Oak Ridge freeze wall fused in January 1998 and contained the same suite of elements present at Fukushima for six years—the duration specified for Fukushima's wall by the Nuclear Regulation Authority panel—until regulators ordered the U.S. Department of Energy to remediate the site. Phillips is confident that it would have operated beyond its 30-year design life.
Joseph Sopko, director of ground freezing at Moretrench, says that Kajima's proposed one-year timeline for installation and freezing is reasonable in light of an installation he managed for a gold mine in northern Ontario in the late 1990s that saw pipes for a two-mile long wall installed in just under one year. The scale for the Fukushima wall, meanwhile, looks downright small compared to a five-mile-long wall proposed for an oil sands operation in Alberta, for which Moretrench is currently conducting pilot studies.
One major drawback, however, is power consumption. While the walls take months or years to thaw once frozen—and are thus immune to power outages—they do require long-term refrigeration to endure. Typically, the cooling power required for maintenance is about half of what was required to form the wall.
Tepco and Kajima could save energy if they employed a technique used at Oak Ridge. Its wall incorporated passive devices known as thermosyphons that Arctic Foundations has installed across Alaska to reinforce melting permafrost under buildings and infrastructure. A coolant gas passively cycles in the tubes whenever the ground is warmer than the air above, absorbing heat at the bottom by boiling, then dumping that heat at the top by condensing and finally dripping back down the tube wall to repeat the cycle.
Thanks to the inclusion of thermosyphons, the Oak Ridge system consumed barely 100,000 kilowatt-hours of power annually—less than 10 homes would use in a year. "It's a very efficient system for moving heat against gravity. There are no moving parts," says Yarmak.
Still, while Yarmak would love to export Arctic Foundations' thermosyphons to Japan, he says power consumption is not a critical issue for Fukushima. Even with the more conventional freeze wall system that Kajima has proposed, whose power consumption would be roughly 250 times larger than at Oak Ridge, the power use still looks small in context. "For the scope of the problem that Japan has, it's not a lot of energy," he says.